1. Field of the Invention
The present invention relates to a spectroscopic characteristics acquisition unit, an image evaluation unit having the spectroscopic characteristics acquisition unit to evaluate an output result by an image forming apparatus, and an image forming apparatus having the image evaluation unit. The spectroscopic characteristics acquisition unit has a spectroscopic sensor array to obtain spectral data of a measurement target as line data to evaluate an image formed on an image bearing medium in an image forming apparatus, and obtains spectroscopy reflectance for the entire image width to conduct measurement of colors in images.
2. Description of the Background Art
Image forming apparatuses such as printers, copiers, facsimile machines, and digital multi-functional machines, and printing machines provided with a communication function are now commercially available. Various types of image forming methods such as electrophotography, inkjet method, heat-sensitive method, or the like are employed for the image forming apparatuses. Further, in the field of production printing, digitalization has been advanced for cut-sheet production printing machines and continuous-sheet production printing machines, and various types of image forming methods such as electrophotography, inkjet method, or the like are on the market. User needs have been shifting from monochrome printing to color printing, in which demand for multi-dimensional imaging and high precision/high density have increased for printed images. Further, with the advancement of service diversification requested by various users, such as printing photo-level high-quality images, printing catalogs, printing customer-specific commercial information in bills or the like, demand for high quality imaging, security of personal information, and accurate color reproductions has also been increasing.
Some technologies for securing high-quality imaging have been applied to image forming apparatuses using electrophotography by conducting a given process for the image forming apparatuses. In one configuration, a concentration sensor is provided to the image forming apparatus to detect a toner image formed on an intermediate transfer member or a photoconductor as a toner concentration before fusing a toner image on a recording medium, and then the toner supply amount is adjusted based on the detection result. Further, in another configuration for checking personal information, instead of using an image forming process, a camera captures an output image, and then differences in information are identified by detecting text recognition and/or image-to-image differences. Further, in another configuration for color reproduction, color patches are output and color values at several points in the color patches are measured by a spectrometer, and then the color is calibrated based on the measurement result. Such technologies may be deployed to cope with fluctuation of image quality in the same page or between different pages by detecting, for example, the entire image area.
Some evaluation methods for evaluating the entire width of image have been disclosed as below. In “MEASUREMENT SYSTEM AND SCANNING DEVICE FOR THE PHOTOELECTRIC MEASUREMENT OF A MEASUREMENT OBJECT PIXEL BY PIXEL” of WO2006/045621 (or U.S. Pat. No. 7,671,992), a plurality of light receiving elements are arranged in a line, and a mechanism to relatively move a measurement object with respect to a detection system is disposed, and spectral characteristics for the entire width of image is measured, in which light-shielding walls is set between light receiving elements to prevent cross talk of reflection light coming from an area of the measurement object.
In “FULL-WIDTH ARRAY SPECTROPHOTOMETER, METHOD FOR FULL-WIDTH SCAN COLOR ANALYSIS OF COLOR INSPECTION OBJECT, AND METHOD FOR PERFECT TRANSVERSE-DIRECTIONAL SCAN COLOR ANALYSIS OF COLOR PRINTING SHEET” disclosed in JP-2005-315883-A, the entire width of image is continuously illuminated by a light source which can emit light having different wavelength, and then reflection light is detected to obtain spectral characteristics for the entire width of image.
In “METHOD AND DEVICE FOR DETECTING CONCENTRATION OF COLOR INK OF PRINTED MATTER” disclosed in JP-2002-310799-A, the entire face of printed image is continuously illuminated by light, then the image concentration at a given area is detected by a line sensor camera, and then the detected image concentration is averaged and compared with the reference image concentration.
In “IMAGE PROCESSOR” disclosed in JP-3566334-B, a document and the original document are scanned for a plurality of times, and then the similarities is determined using logical sum processing for images based on common color information.
In “DEVICE FOR DETECTING DENSITY OF MULTI-COLOR PRINTED IMAGE ON PRINTING SHEET” disclosed in JP-2003-139702-A, the entire area of printed image is illuminated by light, and by using a combination of a charge coupled devise (CCD) having two-dimensionally configured pixels and a diffraction grating or refractive device, spectral characteristics for the entire image can be obtained.
The color of an image may be measured for the entire image width as follows, for example. In one configuration, a plurality of light beams having different wavelength illuminates a measurement target, and an area sensor captures an image. In another configuration, a measurement system and a measurement target are relatively moved while capturing images by a line sensor. In another configuration, a plurality of capturing systems is set and used to detect reflected light reflected from a measurement target using a given limited wavelength.
When image data is obtained by using a given range of wavelength, if some positional deviation may occur for a measurement target between images, color information at each position on the measurement target cannot be measured correctly.
Color information of a plurality of images, which may correspond to different wavelength, may be correctly measured by several methods. For example, in one method, intensity of reflected light obtained at a point of measurement target of each image is compared with reference image such as original image or document data. In another method, based on intensity of reflected light obtained at a point of measurement target of each image, continuous spectral characteristics are estimated using Wiener estimation or the like.
In such methods, however, when different positions are measured as the measurement target in each of printed images, an error may occur in data comparison between the measured data and reference data and the estimation of continuous spectral characteristics, causing measurement precision to deteriorate.
The above-mentioned related arts may have some problems as follows. In WO2006/045621 (or U.S. Pat. No. 7,671,992), the measurement system includes a line sensor, by which color of measurement target image can be measured for the entire width of image, but it may not have a configuration to reduce positional deviation of image obtained at each wavelength.
In JP-2005-315883-A, the reflection light from a measurement target is continuously obtained by illuminating light on the measurement target using a light source which can emit light having different wavelength. However, deviation may occur in the timeline, by which a same portion of measurement target cannot be measured. Even if a plurality of combinations of the light source and light receiving unit is employed for this configuration, positional deviation of measurement target for each image corresponding to different wavelength may occur. Further, even if a plurality of detectors for filtering light using different colors is included in this configuration, positional deviation between images for a plurality of colors may occur.
In JP-2002-310799-A, color information is obtained for the entire width of image, and then the concentration of detected area is averaged to obtain a representative value, but the color profile for the measurement target may not be guaranteed.
In JP-3566334-B, the image of original document and the image of measurement target are compared for each wavelength to determine the difference of original and measurement target, but fluctuation of color quality of between the measurement target such as printed images cannot be determined. Further, even if an image is reproduced from color information of each one of images, obtained one by one, it cannot be determined whether fluctuation of color quality actually occurs to the measurement targets.
In JP-2003-139702-A, the CCD having two-dimensionally configured pixels is used to obtain image data in one direction and spectral data in the other direction so as to measure color information for the entire width of image.
However, because the reading speed of CCD having two-dimensionally configured pixels is too slow compared to a line sensor due to a limitation of data reading performance of CCD, there is a limit for speed to obtain color information of measurement target. As such, as for conventional spectroscopy sensor measuring spectroscopy information of a measurement target for the entire width of image, it is hard to conduct a high-speed reading with a high spectroscopy precision at the same time.
In view of the above-described problems of the conventional art, the inventors of the present invention have proposed a spectroscopic characteristics acquisition unit as disclosed in JP-2010-256324-A, which includes a light emitting unit to emit light onto a measurement target, a spectroscopy sensor to diffract diffuse-reflected light reflected from the measurement target illuminated by the light emitting unit, and a light receiving unit to receive the diffuse-reflected light diffracted by the spectroscopy sensor. The light receiving unit may include a spectroscopic sensor array having a plurality of spectroscopy sensors arranged in one direction. The spectroscopy sensor may include a given number of pixels arranged in one direction, which can be sensitive to different light having different spectral characteristics.
FIG. 1 shows an example configuration of the spectroscopic characteristics acquisition unit of JP-2010-256324-A. The spectroscopic characteristics acquisition unit may include a light source of line type, a line sensor 0-101, a diffraction grating 0-102, an image-focusing optical system 0-103 used as a first focusing unit, a pinhole array 0-104 having a plurality of pinholes, and a Selfoc (registered trademark) lens array 0-105 used as a second focusing unit. Light emitted from the line light source is reflected by an image bearing medium 0-106 (e.g., paper), and the diffuse-reflected light is guided to the pinhole array 0-104 by the Selfoc lens array 0-105. The light exiting from a plurality of pinholes of the pinhole array 0-104 reaches the image-focusing optical system 0-103, is diffracted by the diffraction grating 0-102, and then focused on the line sensor 0-101. The line sensor 0-101 includes a plurality of spectrometer units arranged in one direction, in which one image exiting from one pinhole enters one spectrometer unit.
The selfoc lens array 0-105, used as the second focusing unit (or image-focusing optical unit) and a contact-type focusing device has the following merits: (1) even if a distance period of pinholes of the pinhole array 0-104 and a distance period of elements of the contact-type focusing device are different, the measurement target image can be focused on the pinhole array 0-104; and (2) relative positioning of the contact-type focusing device and the pinhole need not be strictly accurate.
However, in the optical system of FIG. 1, a problem such as the variation or fluctuation of light intensity may occur between individual spectrometers. The difference between the lens distance period of the selfoc lens array 0-105 and the pinhole distance period of the pinhole array 0-104 may cause light exiting angle to fluctuate, and thus the light intensity that can be coupled to the image-focusing optical system fluctuates depending on locations on the selfoc lens array 0-105 and the pinhole array 0-104. Such condition is explained with reference to FIGS. 2A and 2B.
FIG. 2A shows a case when the lens center of one lens of the selfoc lens array and the center of one pinhole are aligned, in which a light path is shown in the upper side of FIG. 2A and the light intensity profile of light emitting from the pinhole at various angles is shown in the lower side of FIG. 2A. In FIGS. 2A and 2B, the shaded portion indicates the light path. Because one pinhole receives light from a plurality of selfoc lenses, the exiting light intensity profile includes a plurality of peaks for different light emitting angles. Because the image-focusing optical system used as the first focusing unit has a finite entrance pupil, an image around the center (exiting angle: 0 degree) can be taken in FIG. 2A.
FIG. 2B shows a condition when the lens center of the selfoc lens array and the center of one pinhole are not aligned with each other. Generally, as for the light intensity profile of light emitting from the pinhole at various angles, light intensity becomes weaker at the center of the exiting light profile and stronger around the center of the exiting light profile as shown in FIG. 2B. As a result, the light intensity that can be taken by the image-focusing optical system, configured as shown in FIG. 2B, becomes small compared to a configuration when the lens center and the pinhole center are aligned as shown in FIG. 2A. As such, when the pinhole distance period of pinhole array and the lens distance period of selfoc lens array are different, fluctuation may occur to intensity of the light that is taken in.
Further, another issue is discussed. In the line-type spectroscopy sensor of FIG. 1, the farther from the center portion of sensing area and the closer to the edge portion of sensing area (i.e., as image height with respect to a focus lens becomes greater), the angle of exiting light, exiting from the pinholes and taken by the image-focusing optical system, becomes more slanted for the light exiting from the portion closer to the edge portion of sensing area as shown in FIG. 1.
For example, as shown in FIG. 3, the light intensity that can be taken by the image-focusing optical system for the light exiting from the edge portion of sensing area becomes small compared to the light intensity of the light exiting from the center portion of sensing area because the angle of exiting light at the edge portion of sensing area becomes greater than the angle of exiting light at the center portion of sensing area. Further, fluctuation of taken-light intensity may occur at other portion between the center portion and the edge portion of sensing area depending on the angle of exiting light. Such issue may still remain even if the lens pitch of selfoc lens array and the pinhole pitch are matched.
As a result, as shown in FIG. 4, the light intensity that can be taken for image height having different height may periodically vary or fluctuate as shown by a solid line, and the light intensity may become smaller for the image having higher image height. FIG. 4 also shows an ideal light intensity profile using a dotted line. The term “image height” means an image height with respect to the image-focusing optical system, and the maximum image height means the greatest image height for the effective sensing area. JP-2010-256324-A is silent about such light intensity reduction phenomenon of light exiting from the edge portion of sensing area.
Further, still another issue is discussed. In FIG. 1, the diffuse-reflected light reflected from the center portion of sensing area in the image bearing medium 0-106 such as paper reflects from the sensing area substantially perpendicular from the image bearing medium 0-106 and eventually reaches the image-focusing optical system, while the diffuse-reflected light reflected from the edge portion of sensing area in the image bearing medium 0-106 is reflected from the sensing area at an angle and eventually reaches the image-focusing optical system at that angle. In general, the reflected light reflected from printed images has angle-specific spectral characteristics, in which the spectral characteristics of light reflected with different angles have different spectral characteristics even if lights are reflected from the same image when the same image is disposed at different positions on the image bearing medium 0-106. Therefore, in the configuration shown in FIG. 1, the spectral characteristics obtained from the center portion of the sensing area and the spectral characteristics obtained from the edge portion of sensing area may become different, and a measurement error may occur due to such angle-specific spectral characteristics.